4 (a) Evaporation and Condensation

Boiling and EvaporationIf you’ve ever poured water into a pot, you’ve seen how the liquid takes the shape of the container. This is true because the molecules in liquid water move freely, bouncing off one another.What happens if you place the pot of water on a stove and heat it? As more energy is added to the liquid water, the speed of the molecules increases and the temperature rises. At 100°C, the water boils and a change of state occurs. The molecules have enough energy to escape the liquid and become invisible water vapor. The molecules in a gas move even more freely than those in a liquid.Another way that liquid water can become a gas is through evaporation. Evaporation is the process by which molecules at the surface of a liquid absorb enough energy to change to the gaseous state. If you let your hair air-dry after going swimming, you are taking advantage of evaporation.CondensationAs water vapor cools down, it releases some of its energy to the surroundings. The molecules slow down and the temperature decreases. As the temperature of the gas reaches 100°C, the water vapor begins to change back to the liquid state. The process by which a gas changes to a liquid is called condensation. When you fog up a window by breathing on it, you are seeing the effects of condensation. The invisible water vapor in your breath is cooled by the window and forms drops of liquid water.FreezingIf those drops of liquid water cooled, the molecules would lose energy. They would start to move more and more slowly. At 0°C, the liquid water freezes, changing into solid ice. If you have ever observed an icicle forming from water dripping off a roof, you have seen this change of state in progress.MeltingSuppose that you put an ice cube in a pot and place it on the stove. As you heated it, the molecules in the ice would start moving faster. The temperature would rise. When the temperature reached 0°C, the solid ice would melt and become liquid water.